When there is more evidence in favor of something than against or when there are reasonable grounds for believing the truth of an assertion, which is something more than mere suspicion.
Instrumentalities of crime:
Items, goods, tools, or anything else used to facilitate commission of a crime. This could include anything from lock-picking sets to software or hardware used by computer hackers.
A sworn, written statement voluntarily made, before a proper authority. Affidavits are commonly used to replace live testimony as a matter of necessity (such as when a witness is no longer available due to death, but has already made out an affidavit) or even as a matter of convenience (such as when it would be difficult to present the witness to the court at the appropriate time).
An out-of-court statement offered into evidence for the purpose of establishing the truth of the matter asserted. Although the rules regarding hearsay are complicated, hearsay is generally not admissible as evidence in a criminal trial because the party against whom the statement is offered does not have the opportunity to cross-examine the declarant (the person who made the statement).
Before a judge or magistrate may issue a search warrant, there must be a finding of probable cause. The level of evidence required to demonstrate probable cause is greater than "mere suspicion," but is less than the level required for a criminal conviction (beyond a reasonable doubt). The facts must demonstrate that a reasonable person would believe that the location, which is the subject of the warrant contains evidence of a crime, the instrumentalities of a crime (e.g., safe-cracking equipment), contraband (e.g., illegal materials such as drugs), or the fruits of a crime (e.g., stolen property). See Carroll v. United States, 267 U.S. 132 (1925).
EXAMPLE (1): Officer Tropvite has seen Joe lurking around the back of a store nearby his home and suspects that Joe plans to rob the store. Although the Officer has no reason to believe that any instrumentalities of crime exist at Joe’s house, he asks Judge Prudent for a search warrant, hoping that the search might scare Joe off from the possible future crime. Without some evidence from which a reasonably prudent person could conclude that tools to be used in the crime would be found in Joe's house, Judge Prudent should not issue the warrant.
EXAMPLE (2): Not to be discouraged from capturing Joe and winning county-wide acclaim, Officer Tropvite follows Joe around for a few days. Over that period of time he observes (and documents) Joe purchasing a large pair of bolt cutters, a black ski mask, leather gloves, a ladder, and 2 large duffel bags. He watches Joe transfer these items from his car to his garage. Returning to Judge Prudent with this new evidence, Officer Tropvite will likely be granted his request for a search warrant for Joe’s garage.
Probable cause may be demonstrated by live, sworn testimony or by affidavit. More importantly, an affidavit based on hearsay (which could not be used as evidence in a criminal trial) can be used as the basis for issuing a search warrant, so long as the circumstances in their totality establish probable cause. See Illinois v. Gates, 462 U.S. 213 (1983).
EXAMPLE (1): Officer Diligent seeks a warrant based on his testimony that Sneak told Diligent that he (Sneak) overheard Warren asking his girlfriend to go buy $500 worth of drugs in cash and to bring them back to his house. Officer Diligent’s testimony to this effect would not be admissible as evidence against Warren in his trial – it is inadmissible hearsay. However, for the purpose of establishing probable cause such that a search warrant may be issued, Officer Diligent’s testimony may be admissible. If we add Diligent’s testimony that he then witnessed the girlfriend going to an ATM, withdrawing $500 in cash and then returning to the house one hour later, the totality of the circumstances may be sufficient to issue the warrant.
EXAMPLE (2): Officer Techie e-mails Judge Conservative and requests a search warrant for a storage locker that he believes contains stolen salmon. He also sends an attachment with the e-mail that contains digital pictures of masked men unloading the salmon into the locker. The picture is time-stamped by the camera, which records the time as 2:37 a.m. Although the evidence might be sufficient for a reasonably prudent person to conclude that the fruits of a crime are located within the locker, because there has been no sworn testimony to that effect and no affidavit has been written (the e-mail is not a sworn writing made before a proper authority), Judge Conservative should deny the Officer Techie's request.